X-ray Absorption Spectroscopy Measurements Research Articles

Absorption Correction for 3D Elemental Distributions of Dental Composite Materials Using Laboratory Confocal Micro-X-ray Fluorescence Spectroscopy.

Confocal micro-X-ray fluorescence (micro-XRF) spectroscopy facilitates three-dimensional (3D) elemental imaging of heterogeneous samples in the micrometer range. Laboratory setups using X-ray tube excitation render the method accessible for diverse research fields but interpretation of results and quantification remain challenging. The attenuation of X-rays in composites depends on the photon energy as well as on the composition and density of the material. For confocal micro-XRF, attenuation severely impacts elemental distribution information, as the signal from deeper layers is distorted by superficial layers. Absorption correction and quantification of fluorescence measurements in heterogeneous composite samples have so far not been reported. Here, an absorption correction approach for confocal micro-XRF combining density information from microcomputed tomography (micro-CT) data with laboratory X-ray absorption spectroscopy (XAS) and synchrotron transmission measurements is presented. The energy dependency of the probing volume is considered during the correction. The methodology is demonstrated on a model composite sample consisting of a bovine tooth with a clinically used restoration material.

Co-Ni co-sputter deposited bimetallic thin film catalysts for alkaline hydrogen and oxygen evolution reactions

Co-Ni bi-metallic thin films with different stoichiometries have been deposited by magnetron co-sputtering technique on Ni foam (NF), Si and carbon paper substrates. The structural and morphological characterisations of these films have been done by X-ray diffraction, X-ray absorption spectroscopy and Field Emission Scanning Electron Microscope measurements. The performances of these films for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been evaluated through electrochemical measurements like Linear Sweep Voltammetry, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy measurements. Operando X ray absorption near edge spectroscopy measurements have also been carried out during HER and OER to decipher the redox reaction mechanisms. It has been observed that the HER and OER performance of all the Co Ni bi-metallic thin films on NF are found to be better than that of bare NF. The CoNi sample with composition of 58.7 % Co and 41.3 % Ni shows the best catalytic activity and stability towards HER and OER reactions. Though the HER activity is not the best but is comparable to many results reported in the literature, however the catalyst shows excellent OER activity with overpotential of 214 mV for 10 mA/cm2 current density. This is much better than that of the OER benchmark catalyst RuO2 and most of those reported in the literature. The OER is kinetically slower than HER and requires more overpotential, hence a significant improvement in the catalytic OER activity will definitely enhance the overall water splitting activity and hydrogen production efficiency. Thus the Co-Ni bi-metallic catalyst prepared by an easily scalable and highly reproducible magnetron co-sputtering technique has significant potential to emerge out as a technologically viable water splitting catalyst.

Nanostructure and dynamics of N-truncated copper amyloid-β peptides from advanced X-ray absorption fine structure.

An X-ray absorption spectroscopy (XAS) electrochemical cell was used to collect high-quality XAS measurements of N-truncated Cu:amyloid-β (Cu:Aβ) samples under near-physiological conditions. N-truncated Cu:Aβ peptide complexes contribute to oxidative stress and neurotoxicity in Alzheimer's patients' brains. However, the redox properties of copper in different Aβ peptide sequences are inconsistent. Therefore, the geometry of binding sites for the copper binding in Aβ4-8/12/16 was determined using novel advanced extended X-ray absorption fine structure (EXAFS) analysis. This enables these peptides to perform redox cycles in a manner that might produce toxicity in human brains. Fluorescence XAS measurements were corrected for systematic errors including defective-pixel data, monochromator glitches and dispersion of pixel spectra. Experimental uncertainties at each data point were measured explicitly from the point-wise variance of corrected pixel measurements. The copper-binding environments of Aβ4-8/12/16 were precisely determined by fitting XAS measurements with propagated experimental uncertainties, advanced analysis and hypothesis testing, providing a mechanism to pursue many similarly complex questions in bioscience. The low-temperature XAS measurements here determine that CuII is bound to the first amino acids in the high-affinity amino-terminal copper and nickel (ATCUN) binding motif with an oxygen in a tetragonal pyramid geometry in the Aβ4-8/12/16 peptides. Room-temperature XAS electrochemical-cell measurements observe metal reduction in the Aβ4-16 peptide. Robust investigations of XAS provide structural details of CuII binding with a very different bis-His motif and a water oxygen in a quasi-tetrahedral geometry. Oxidized XAS measurements of Aβ4-12/16 imply that both CuII and CuIII are accommodated in an ATCUN-like binding site. Hypotheses for these CuI, CuII and CuIII geometries were proven and disproven using the novel data and statistical analysis including F tests. Structural parameters were determined with an accuracy some tenfold better than literature claims of past work. A new protocol was also developed using EXAFS data analysis for monitoring radiation damage. This gives a template for advanced analysis of complex biosystems.

A sub-100 nm thickness flat jet for extreme ultraviolet to soft X-ray absorption spectroscopy.

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.

Photocatalytic Activity of BaAl2O4 for Water Purification.

Degradation of dyes under natural light sources is one of the most active research areas in basic science for greener technology. In this context, the photocatalytic activity of semiconductors has received massive attention in solving water treatment-related issues as these possess enormous potential for degrading organic impurities. Here, we report that barium aluminate (BaAl2O4, BAO), which has been extensively studied for photoluminescence applications, is found to be a highly potent candidate for photocatalytic activities. We have explored the degradation of dyes (meant for water purification) by using the photocatalytic properties of pure and Dy- and Yb-codoped BAO. Crystal structure, electron microscopy, and Raman analysis of the autocombustion-synthesized pure and codoped BAO samples revealed significant morphological changes such as increased particle size and stabilization of rod-like structures. UV-vis absorbance measurements confirm the presence of multiple bandgaps in the BAO samples, which is substantiated by X-ray absorption spectroscopy measurements. Photocatalytic degradation studies of methylene blue (MB) dye (with different catalyst concentrations, dopings, and MB dye concentrations) have been carried out by using BAO. The kinetics of the photocatalytic degradation measurements has been explained by the Boltzmann distribution function, and the fastest (in less than 40 min), with more than 99% degradation of MB impurity, is reported here for the first time in BAO compounds. Synthesized BAO samples show excellent cyclic stability, which is essential for their potential applications in environmental remediation. The trade-off between the enhancement of surface area and increased particle size is considered the key parameter for controlling the photocatalytic performance of the BAO catalyst after Dy and Yb codopings.

Unveiling the non-innocence of vanadium dopant in TiO2 nanocrystals for advanced energy storage and smart windows

Vanadium doped TiO2 NCs stand out as a promising candidate for energy storage applications due to its high electrical conductivity and redox properties. However, the thermodynamical behavior of the material under working conditions has not been explored and the reasons for its superior performance remain unlocked. This study explores the use of a combination of advanced in situ spectroscopy techniques, including x-ray absorption spectroscopy (XAS), spectro-electrochemistry (SEC), and electrochemical impedance spectroscopy (EIS) to provide unprecedented insights into the intricate electrochemical reaction mechanisms within these nanocrystals. Density functional theory calculations and EIS reveal the active role of substitutional V ions in the TiO2 anatase network as electron donors, enhancing surface charge and carrier density and improving pseudocapacitive properties. Cyclic voltammetry and in situ SEC reveal that V-doped TiO2 NCs exhibit significantly improved charge storage capacities, particularly in the pseudo-capacitance storage mechanism. In situ SEC and XAS analyses indicate that a more effective reduction of Ti4+ ions occurs during the electrochemical process in doped NCs, leading to higher charge capacitance and faster processes. Furthermore, in situ XAS measurements of the V K-edge revealed that the vanadium ions, beyond improving the redox behavior of the host, also actively participate in the reduction process. The significant changes in the V K-edge XANES and extended x-ray absorption fine structure spectra observed under reduction conditions can be ascribed to a change in the structure and oxidation state of the vanadium ions during the electrochemical reaction.

Strong signature of uncompensated magnetization in frustrated cobalt manganites using x-ray magnetic circular dichroism study

The present study is focused on the investigation of the distorted tetragonal phase of mixed spinel oxides, due to their technological relevance in the field of electronics, spintronics, magnetism, catalysis, and electrochemical energy storage. Herein, we report on solgel synthesized multivalent cobalt manganites, CoMn2O4 (CMO), and subjected them to a comprehensive analysis to elucidate their physicochemical characteristics at room temperature. Analysis employing powder x-ray diffraction patterns and electron microscopy (including field-emission scanning electron microscopy and high-resolution transmission electron microscopy) results confirmed the formation of a pure and exceptionally crystalline, distorted tetragonal phase of mixed CMO. Synchrotron-based x-ray absorption spectroscopic (XAS) measurements in the total electron yield mode examined local electronic structures affirming the formation of CMO with uncompensated electronic states involving Co2+, Co3+, Mn2+, Mn3+, and Mn4+ cations. Concurrently, XAS and x-ray magnetic circular dichroism analyses revealed antiferromagnetic coupling within Co and Mn sublattices in CMO, indicating the presence of uncompensated electronic states. Vibrating sample magnetometry results demonstrated clear hysteresis behavior, explicitly indicating the coexistence of super-paramagnetic and canted antiferromagnetic characteristics in CMO, as validated through the Langevin function fitting and x-ray magnetic circular dichroism results. The noticeable absence of saturated magnetization confirmed the high degree of spin canting, primarily stemming from the presence of the Yafet–Kittel spin arrangement.

Li+ Cations Activate NiFeOOH for Oxygen Evolution in Sodium and Potassium Hydroxide.

The efficiency of electrolysis is reduced due to the sluggish oxygen evolution reaction (OER). Besides catalyst properties, electrocatalytic activity also depends on the interaction of the electrocatalyst with the electrolyte. Here, we show that the addition of small amounts of Li+ to Fe-free NaOH or KOH electrolytes activates NiFeOOH for the OER compared to single-cation electrolytes. Moreover, the activation was maintained when the solution was returned to pure NaOH. Importantly, we show that the origin of activation by Li+ cations is primarily non-kinetic in nature, as the OER onset for the mixed electrolyte does not change and the Tafel slope at low current density is ~30 mV/dec in both electrolytes. However, the increase of the apparent Tafel slope remains lower at increasing current densities in the presence of Li+. Based on electrochemical quartz crystal microbalance and in situ X-ray absorption spectroscopy measurements, we show that this reduction of non-kinetic effects is due to enhanced intercalation of sodium, water and hydroxide. This enhanced electrolyte penetration facilitates the OER, especially at higher current densities and for increased catalyst loading. Our work shows that mixed electrolytes where distinct cations can have different roles provide a simple and promising strategy towards improved OER rates.

Using XAS to monitor radiation damage in real time and post-analysis, and investigation of systematic errors of fluorescence XAS for Cu-bound amyloid-β.

X-ray absorption spectroscopy (XAS) is a promising technique for determining structural information from sensitive biological samples, but high-accuracy X-ray absorption fine structure (XAFS) requires corrections of systematic errors in experimental data. Low-temperature XAS and room-temperature X-ray absorption spectro-electrochemical (XAS-EC) measurements of N-truncated amyloid-β samples were collected and corrected for systematic effects such as dead time, detector efficiencies, monochromator glitches, self-absorption, radiation damage and noise at higher wavenumber (k). A new protocol was developed using extended X-ray absorption fine structure (EXAFS) data analysis for monitoring radiation damage in real time and post-analysis. The reliability of the structural determinations and consistency were validated using the XAS measurement experimental uncertainty. The correction of detector pixel efficiencies improved the fitting χ2 by 12%. An improvement of about 2.5% of the structural fitting was obtained after dead-time corrections. Normalization allowed the elimination of 90% of the monochromator glitches. The remaining glitches were manually removed. The dispersion of spectra due to self-absorption was corrected. Standard errors of experimental measurements were propagated from pointwise variance of the spectra after systematic corrections. Calculated uncertainties were used in structural refinements for obtaining precise and reliable values of structural parameters including atomic bond lengths and thermal parameters. This has permitted hypothesis testing.

Multiple Metal-Nitrogen Bonds Synergistically Boosting the Activity and Durability of High-Entropy Alloy Electrocatalysts.

The development of Pt-based catalysts for use in fuel cells that meet performance targets of high activity, maximized stability, and low cost remains a huge challenge. Herein, we report a nitrogen (N)-doped high-entropy alloy (HEA) electrocatalyst that consists of a Pt-rich shell and a N-doped PtCoFeNiCu core on a carbon support (denoted as N-Pt/HEA/C). The N-Pt/HEA/C catalyst showed a high mass activity of 1.34 A mgPt-1 at 0.9 V for the oxygen reduction reaction (ORR) in rotating disk electrode (RDE) testing, which substantially outperformed commercial Pt/C and most of the other binary/ternary Pt-based catalysts. The N-Pt/HEA/C catalyst also demonstrated excellent stability in both RDE and membrane electrode assembly (MEA) testing. Using operando X-ray absorption spectroscopy (XAS) measurements and theoretical calculations, we revealed that the enhanced ORR activity of N-Pt/HEA/C originated from the optimized adsorption energy of intermediates, resulting in the tailored electronic structure formed upon N-doping. Furthermore, we showed that the multiple metal-nitrogen bonds formed synergistically improved the corrosion resistance of the 3d transition metals and enhanced the ORR durability.

Effect of sintering temperature on structural, elastic, and hyperfine features of nickel ferrite nanoparticles

Temperature is one of the key parameters in the synthesis of a system of magnetic nanoparticles and can affect considerably the arrangement of atoms in the crystal structure. Consequently, the magnetic, elastic, and structural properties are strongly impacted. In the magnetically soft NiFe1.8O4 spinel ferrite, the relative number of Ni and Fe ions in each of the two possible different geometries, tetrahedral (T) and octahedral (O), is of extreme importance. In this work, this material was studied on products of co-precipitation synthesis followed by a temperature dependent sintering. We performed 57Fe Mössbauer spectroscopy (MS), X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM and TEM), X-ray absorption spectroscopy (XAS) using synchrotron radiation at Ni K- and L2-edge, and vibrating sample magnetometry (VSM) measurements. From XAS measurements it was found that Ni atoms prefer an octahedral coordination when the sintering temperature is increased. MS showed that the T-sites are more populated by Fe ions than the O-sites, a fact that can be explained by the presence of Fe ion vacancies in the O- sites. However, as the sintering temperature increases, we found that there is also a migration of Fe atom to the O-sites. Additionally, based on MS measurements in 78–292 K region, we estimated the Debye temperatures of the Fe ions in the tetra- and octahedral geometries and found that the recoiless f-factor depends on the sintering temperature of the nickel ferrite powders. The ratio fO/fT is about 0.98 and 0.95 at room temperature for the samples sintered at 1000 °C and 600 °C, respectively.

Development of a microwave chemical cell coupled with post-wall waveguide for in situ/operando soft x-ray absorbance spectroscopy under microwave irradiation

In this study, we developed a microwave chemical cell for total electron yield soft x-ray absorption spectroscopy (XAS) using synchrotron radiation under microwave irradiation. In addition, in situ XAS measurements of ammonia borane were performed using the device developed. The device comprises a post-wall waveguide housing a sample holder formed within it. Electrons emitted from the sample during soft x-ray irradiation pass through the metal plates of the waveguide and are measured using a picoammeter. Integration of the microwave irradiation structure into a synchrotron radiation beamline, which is a challenge in soft x-ray analysis requiring measurements in a high-vacuum atmosphere, was achieved by miniaturizing the device using 24.125 GHz ISM band microwaves, instead of the typical 2.45 GHz microwaves. Performance of the device was thoroughly assessed through temperature distribution simulation, temperature measurement of water, and evaluation of frequency characteristics. When the sample holder was filled with water and irradiated with 3 W microwaves, the temperature increased to 97 °C, which is close to the boiling point. The in situ XAS measurements performed using this device with ammonia borane/graphite (2:1) powder at the B-K edge during microwave irradiation showed that peaks associated with hydrogen desorption intensified with prolonged microwave exposure. This result indicates that microwave heating in a vacuum and soft x-ray analysis during microwave heating were achieved. The developed microwave chemical cell emerges as a powerful tool, facilitating advancements in our understanding of elementary chemical processes and elucidating microwave-specific effects such as local heating during microwave irradiation.

Increasing Al-Pair Abundance in SSZ-13 Zeolite via Zeolite Synthesis in the Presence of Alkaline Earth Metal Hydroxide Produces Hydrothermally Stable Co-, Cu- and Pd-SSZ-13 Materials

Replacing alkaline for alkaline-earth metal hydroxide in the synthesis gel during the synthesis of siliceous SSZ-13 zeolite (Si/Al~10) yields SSZ-13 with novel, advantageous properties. Its NH4-form ion-exchanges higher amount of isolated divalent M(II) ions than the conventional one: this is the consequence of an increased number of Al pairs in the structure induced by the +2 charge of Sr(II) cations in the synthesis gel that force two charge-compensating AlO4− motives to reside closer together. We characterize the +2 state of Co(II) ions in these materials with infra-red spectroscopy and X-ray absorption spectroscopy measurements and show their utility for NOx pollutant adsorption from ambient air: the ones derived from SSZ-13 with higher Al pair content contain more isolated cobalt(II) and, thus, perform better as ambient-air NOx adsorbers. Notably, Co(II)/SSZ-13 with an increased number of Al pairs is significantly more hydrothermally stable than its NaOH-derived analogue. Loading Pd(II) into Co-SSZ-13(Sr) produces an active NOx adsorber (PNA) material that can be used for NOx adsorption from simulated diesel engine exhaust. The critical issue for these applications is hydrothermal stability of Pd-zeolites. Pd/SSZ-13 synthesized in the presence of Sr(OH)2 does not lose its PNA capacity after extremely harsh aging at 850 and 900 °C (10 h in 10% H2O/air flow) and loses only ~55% capacity after hydrothermal aging at 930 °C. This can be extended to other divalent metals for catalytic applications, such as copper: we show that Cu/SSZ-13 catalyst can survive hydrothermal aging at 920 °C without losing its catalytic properties, metal dispersion and crystalline structure. Thus, we provide a new, simple, and scalable strategy for making remarkably (hydro)thermally stable metal-zeolite materials/catalysts with a number of useful applications.

Enhanced Electrochemical Performance of MnCo1.5Fe0.5O4Spinel for Oxygen Evolution Reaction through Heat Treatment.

MnCo1.5Fe0.5O4 spinel oxide was synthesized using the sol-gel technique, followed by heat treatment at various temperatures (400, 600, 800, and 1000 °C). The prepared materials were examined as anode electrocatalysts for water-splitting systems in alkaline environments. Solid-state characterization methods, such as powder X-ray diffraction and X-ray absorption spectroscopy (XAS), were used to analyze the materials' crystallographic structure and surface characteristics. The intrinsic activity of the MnCo1.5Fe0.5O4 was fine-tuned by altering the electronic structure by controlling the calcination temperature, and the highest activity was observed for the sample treated at 800 °C. A shift in the valence state of surface cations under oxidative conditions in an alkaline solution during the oxygen evolution reaction was detected through ex situ XAS measurements. Moreover, the influence of the experimental conditions on the electrocatalytic performance of the material, including the pH of the electrolyte and the temperature, was demonstrated.

Electrochemical/computational analysis and the derivation of Pourbaix diagrams of redox-active structural and adsorbed species in uranyl peroxide clusters (U60)

Electrochemical analysis of U60 nanoclusters (Li40K20[UO2(O2)(OH)]60(H2O)214) and their natural analog, the mineral studtite (UO2)O2(H2O)2·(H2O)2, was carried out using cyclic voltammetry with a powder microelectrode setup. Voltammetric analysis was supplemented by ab initio quantum mechanical modeling to understand the mechanism, thermodynamics, and structural changes during redox transition (with a semi-quantitative kinetics evaluation). This research aims to better understand the redox behavior and transport of dissolved and structural (incorporated) uranyl units in the environment. Further analysis is performed to determine if redox switching of actinyl ions in clusters can be performed while leaving the cluster intact or at what redox transition it disintegrates. Quantum-mechanical calculations were applied to approximate electrochemical peak potential shifts and add them to standard reduction potentials to more accurately assign peaks to specific redox transitions between different species. These peak potentials are calculated from oxidation state-dependent binding energies between different uranyl species either within or adsorbed to U60. This theoretical approach incorporates extensive error cancellation and serves to predict electrochemical redox potentials and identify the reaction taking place. Voltammograms of U60 in electrolyte solutions exhibit kinetically-inhibited coupled redox peaks assigned to the U(VI)/U(V) transition of cluster structural uranyl units at -0.34V (vs. standard hydrogen electrode), indicative of U60 reduction from UO22+ to UO2+. The U(V)/U(IV) transition at -0.71V is assigned to the subsequent U60 reduction of UO2+ to UO20. A method of observing these transitions in situ using synchrotron x-ray absorption spectroscopy was explored. Voltammetry, modeling, and peak analysis indicate a kinetically-inhibited two-step, one-e- (per step) reduction of U60 from U(VI) to U(V) to U(IV). Voltammetry and x-ray absorption spectroscopy measurements performed before and after exhaustive cycling and the highly stable nature of U60 provide evidence that clusters typically do not break apart upon reduction. Voltammograms of U60 collected in varied uranyl solutions are indicative of UO22+ adsorption to the cluster surface and its subsequent reduction at +0.24V (U(VI)/U(V)) and -0.05V (U(V)/U(IV)). In uranyl solutions in electrolyte, the two-step, one-e- (per step) reduction of U60 is superseded by uranyl adsorption from solution to the electrode surface with subsequent reduction via U(V) disproportionation. This study represents the sole electrochemical investigation of U60 in the literature and only the second of any uranyl peroxide nanocluster. It is also one of the first to use quantum-mechanical approaches in combination with existing solution-based standard reduction potentials to calculate Pourbaix (EH-pH) diagrams for adsorbed and structural species.

Effect of Dissolved Gases on Pt Nanoparticle Catalysts Investigated By in Situ SAXS, XAS Measurements

Understanding of the structural and the chemical properties of Pt catalysts in polymer electrolyte fuel cells (PEFCs) is necessary to improve the activity for the durability of the catalysts. The small-angle X-ray scattering (SAXS) and the X-ray absorption spectroscopy (XAS) measurements are widely used to investigate these properties. We developed a system that combines in situ A-SAXS/XAS measurements in the same field of observation using a channel flow electrode (CFE) cell and characterized the chemical states, average diameter and size distributions of Pt catalysts under controlled electrochemical conditions. In this study, we investigated an effect of dissolved gases on Pt nanoparticle catalysts using the system.All experiments were carried out at SPring-8 BL19B2. A commercial Pt/carbon black catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo) was used as test electrode in this experiment. Prior to SAXS and XAS measurements, the electrochemical potential was cycled between 0.6 and 1.0 V vs. the RHE in O2 or N2-saturated 0.1 M HClO4. For the XAS measurements, the Pt LⅢ edge was used. The ionization chamber and a seven-elements silicon drift detector were used to obtain the XAS spectra. For the SAXS measurements, the X-ray energy was set to 11.5 keV and 11.55 keV, which are below and near to the Pt LIII absorption edge, respectively. The scattered X-ray was detected using an area detector (PILATUS 2M). Figure 1 show the X-ray absorption near edge structure (XANES) spectra of the Pt catalyst after the potential cycles in O2-saturated 0.1 M HClO4. The white-line peak intensity increased and became higher at a higher potential cycle. This indicates the partial electron-transfer from Pt atoms or initial stage of Pt oxidation. We also will discuss the effect of dissolved gases for morphological and chemical states from SAXS and XAS data. Figure 1

Development of Novel High Capacity Fluoride Ion Battery Cathode Utilizing Anion Redox of Sulfur

All-solid-state fluoride-ion batteries (FIBs), which are based on the shuttling of fluoride ions, have been considered as one of the promising alternates for next-generation energy storage devices because of high theoretical energy density and safety. Benefiting from the multi-covalent fluorination process, metal/metal fluorides (M/MFx) have been firstly utilized as electrode materials for FIBs with high theoretical energy densities.[1] However, M/MFx systems suffer from exceptionable volume expansion upon fluorination process due to the close-packed metal atoms, which would cause blockage of the F- diffusion pathway, resulting in devastating damage to the electrode-electrolyte interfaces. To solve these problems, cathode materials that utilize topotactic fluoride ion intercalation reactions, similar to the electrode materials applied in lithium-ion secondary batteries, are being developed.[2] Cathode materials that utilize topotactic intercalation of fluoride ions are often composed of heavier elements than the cathodes of lithium-ion batteries that use the same type of reaction mechanism, and effective anion redox is required if capacity is to be increased beyond that of lithium-ion secondary batteries.In this research, we investigated Sr2F2Fe2OS2 (SFFOS) compound with a layered structure as a novel cathode material for all-solid-state FIBs, and evaluated its electrochemical properties and elucidated the charge compensation mechanism using anion redox of sulfur.SFFOS was synthesized by a solid-state reaction under vacuum environment using SrF2, SrO, Fe, and S.[3] SFFOS/La0.9Ba0.1F2.9/vapor grown carbon fiber (VGCF) was used as the composite cathode, La0.9Ba0.1F2.9 as the electrolyte and Pb/PbF2/ La0.9Ba0.1F2.9/VGCF as the composite anode to construct the electrochemical cell. Charge-discharge measurements were carried out in the cut-off voltage range of -1.5 to 1.5 V at 140°C. Ex-situ X-ray Diffraction (XRD), X-ray absorption spectroscopy (XAS) measurements of Fe K-edge, S K-edge, O K-edge and F K-edge were carried out under different state of charge during the 1st and the 2nd cycle.SFFOS showed reversible intercalation/de-intercalation of fluoride ions and a capacity of ~6.1 fluoride ions per unit cell (403 mAh g-1). The charge compensation mechanism was revealed for SFFOS by XAS measurements, where S redox contributed to the whole voltage range from -1.5 V to 1.5 V vs. Pb/PbF2, and Fe+2/+3 redox contributed from the middle SOC. The S-S dimers were formed upon charging, which was similar to the trapped O2 molecules observed in lithium-excess metal oxide during charge process[4]. Not only can the F- anions insert around Fe to form Fe-F bonds, but the excess F- anions can also occupy the Sr-S interstitial layers in the original lattice. Compared to the Fe-based layered oxide Sr3Fe2O5, the Fe-based oxysulfide SFFOS showed a lower average voltage but a higher specific capacity. We believe that this study could provide a new understanding of sulfur-based charge compensation and electrochemical fluorination reactions. Reference s : [1] D. Zhang, K. Yamamoto, Y. Uchimoto et al., J. Mater. Chem. A, 2021, 9, 406–412.[2] Y. Wang, K. Yamamoto, Y. Uchimoto, et al., Chem. Mater., 2022, 34, 609–616.[3] Kabbour H, Janod E, Corraze B, et al ., J. Am. Chem. Soc., 2008, 130(26): 8261-8270.[4] R. A. House, P. G. Bruce, et al., Nat. Energy 2020, 5, 777–785. Figure 1

Intercalation Reaction Mechanism of Fluoride-Ions in (Ca, Sr)FeO2 Cathodes with Infinite Layer Structure for All-Solid-State Fluoride-Ion Batteries

Fluoride ion batteries, which use fluoride ions as carriers, are attracting attention as the next generation of storage batteries because of their theoretical high energy density. Conventional research has focused on metal/metal fluorides such as Cu/CuF2 as cathode materials.[1] However, these cathodes have disadvantages of the power density and cyclability due to the rapid decrease in electronic conductivity and large volume change during fluorination and defluorination. To solve these problems, cathode materials that utilize topotactic fluoride ion intercalation reactions, similar to electrode materials applied in lithium-ion secondary batteries, are being developed[2]. However, these materials have the disadvantage of relatively small capacity, compared to metal/metal fluorides.In this study, we focused on SrFeO2and Ca-doped SrFeO2 cathodes with infinite layer structure as new high-capacity intercalation cathodes, and evaluated its electrochemical properties and clarified the charge compensation mechanism.Ca1-xSrxFeO2 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) was synthesized by a low-temperature reduction method using CaH2. Ca1-xSrxFeO2/La0.9Ba0.1F2.9/vapor grown carbon fiber (VGCF) was used as the composite cathode, La0.9Ba0.1F2.9 as the electrolyte and Pb/PbF2/ La0.9Ba0.1F2.9/VGCF as the composite anode to construct the electrochemical cell of the piezoelectric material. Charge-discharge measurements were carried out in the cut-off voltage range of -1.5 to 3.0 V at 140°C. X-ray absorption spectroscopy (XAS) measurements of Fe K-edge, O K-edge, and F K-edge and resonant inelastic X-ray scattering (RIXS) measurements of O K-edge were performed on the samples after charging and discharging.All the Ca1-xSrxFeO2 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) cathode showed voltage plateaus around -0.5 V and 1.5 V during charging, and the maximum discharge capacity (580 mAh g-1) was obtained when x = 0.8, and the cathodes could be repeatedly charged and discharged. XAS of the Fe K-edge revealed that Fe compensated for the charge in the flat potential region around -0.5 V. XAS and RIXS measurements of the O K-edge revealed that the charge compensation was carried out by the formation of O2 molecules (oxidation of oxide ions), as which is observed in lithium-excess metal oxide on charge process[4], in the voltage plateau around 1.5 V. Using inexpensive Ca and Fe, we succeeded in developing an intercalation cathode material that significantly exceeds the capacity of conventional lithium-ion battery cathodes.

Lithium Dendrite Formation inside Li3PS4 Solid Electrolyte Observed Via Multimodal/Multiscale Operando X-Ray Computed Tomography

Sulfide-based all-solid-state batteries using a lithium metal anode are expected to be next-generation batteries due to their extremely high energy density. In order to use the lithium metal as the anode, suppressing dendrite of lithium metal during charge/discharge processes is essentially important. It has been reported that lithium dendrite formation occurs not from the lithium/sulfide solid electrolyte interface, but in the sulfide solid electrolyte, isolated from the interface1, 2. The formation of lithium dendrite within the sulfide solid electrolyte is caused by electron conduction in the sulfide solid electrolyte and at the sulfide solid electrolyte/void interface3. However, fundamental information on the mechanism of lithium dendrite formation in a sulfide solid electrolyte caused by its electron conduction is lacking. In this study, the three-dimensional morphological changes of the lithium dendrite in Li3PS4, which is a typical sulfide solid electrolyte, were observed directly using multimodal/multiscale operando computed tomography (CT) under an applied pressure.Li/Li3PS4/Li cells were constructed in a diameter of 10 mm and 1 mm for the critical current density measurements and X-ray CT measurements by using SPring-8 BL20XU respectively. X-ray CT images for behavior change with the electrochemical operation were collected in micro and nano scales at 25 °C every 30 mins. After a series of data processing steps, these images were converted to cross-sectional slices that were then stacked together to render a 3D reconstruction of the cell. The 3D imaging data coupled with precise species segmentation show that the lithium metal deposition start point is spatially separated from the lithium metal anode. The gradient in thickness of a lithium filament with repeated charging, widening the plating-susceptible region horizontally in the process and eventually led to cell failure. The lithium nucleation initiates along the pre-existing voids where local electronic conductivities are high during the plating. The deposition then widens from the nucleation across the electrolyte horizontally. Accompanied with streak fracture widening through the Li3PS4, does a Li/Li3PS4/Li cell finally short circuit. By combining the multimodal/multiscale operando X-ray computed tomography with X-ray absorption spectroscopy and electrochemical impedance spectroscopy measurements, we revealed that the electronic conduction of reductive decomposition products and the solid electrolyte/void interface cause the lithium deposition within the Li3PS4. These results suggests that the suppression of reductive decomposition and sulfide solid-state electrolytes with low electronic conductivity plays significant roles in suppressing the growth of lithium dendrites in the solid-state electrolyte layer.

Evaluate the Limits of F Covalent Bonding with Transition Metals at High Valent States in Li2MO2f ( M = Mn, Co, Ni ) Oxyfluoride

The capacity of transition metal oxides as Li-ion battery cathodes are limited by instabilities that arise when high states of charge are achieved1. Oxyfluorides with a disordered rock-salt structure have emerged as attractive alternatives2, but the role of F in their electrochemical function, particularly when metals reach high formal oxidation states, remains to be ascertained so far. In our recent study3, using X-ray Absorption Spectroscopy (XAS) measurements of Mn, O and F, we revealed the existence of Mn-F covalent interactions in Li2MnO2F. The results challenged the assumption of F as largely a spectator ion, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggested the existence of unique knobs of design of battery cathodes in these chemical spaces, by manipulating the covalent interactions between transition metals and two different anions. To further expand the understanding of F participation in the covalent bonding and its electrochemical effect on oxyfluoride materials, we synthesized Li2CoO2F and Li2NiO2F4. Solid fluorides of ions like Co(IV) and Ni(IV) are known to be aggressive oxidizers5. Analysis of the transition metal fluoride literature reveals that oxidation states of IV and higher could lead to unstable phases for late transition metals, particularly Co or Ni5. Unlike oxides, rather than reductively releasing F2, they have a strong propensity to act as highly oxidizing F- donors6,7, being able to oxidize even other halogen cations to their VII state. Yet the oxidation of Ni(II) features prominently in oxyfluoride cathodes that were recently discovered. This puzzle and the exact role of F in modulating the formal redox chemistry of a late metal like Ni and Co in the presence of O remains to be elucidated. To address this question and determine a periodic trend on the role of a mixture of anions in improving the energy density in such cathode materials, we conducted a deep dive into Li2CoO2F and Li2NiO2F. We interrogated the covalent interaction between the oxygen 2p states, fluorine 2p states, and the transition metal 3d orbitals, and their respective contribution to the charge compensation mechanism using XAS. These two oxyfluorides data were compared with the previous Li2MnO2F data to understand the effect of varying the transition metal and how that affects the overall electrochemistry of these materials. We also have estimated the M-O and M-F hybridization and provided a periodic trend. This study allowed us to define a rule to manipulate each element in a oxyfluoride that determines the electrochemical properties of these cathode materials.